WC6 - Recap Elementary Particles Recap Elementary Particles...

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Unformatted text preview: Recap Elementary Particles Recap Elementary Particles Define the following particles: baryons, mesons, hadrons, leptons, fermions, bosons. What are in today’s science the basic bricks of matter ? Describe the Standard Model of bricks and force carriers. How do experimentalists look for a certain new particle ? How can experimentalists see the Universe at a very small scale (say 10­16 cm) ? What are the components of a high­energy physics experiment ? What are the two types of experiments in high­energy physics ? How deep can we see today ? Why today’s standard model cannot be the final The Standard Clasification The Standard Clasification Leptons Anti-leptons Quarks Baryons “Bricks” Anti-quarks Mesons Anti-baryons Anti-mesons Hadrons and Anti-hadrons Force Carriers foton weakon gluon graviton Force weak colour gravitational Acts on all charged particles all particles quarks all particles electromagnetic The Second Unification The Second Unification 1st unification was electromagnetism in the 19th century. 2nd unification involved electromagnetism and the weak force No similarities between electromagnetism (electrical wires, the compass) and the weak nuclear force (the disintegration of nuclei or of heavy leptons). No similarities between their carriers: the photon is neutral and with zero mass, the weakon is usually electrically charged and has a large mass (about 50GeV) But in spite of all these differences the two forces were proven to be closely related. The carrier of the weak nuclear force (I) The two diagrams suggest two different compositions for the W+ weakon: positron/neutrino for the first and u/d for the second weak interaction. d quark in neutron eelectronic neutrino virtual weakon (W +) u quark in proton virtual weakon (W +) electronic neutrino positron electronic neutrino e- The carrier of the weak nuclear force (II) W+ ud e ud e ud e Each weakon changes its composition depending on circumstances. This magic behaviour (justifies the name “universal alchemist”) allows the weak force to act on all the particles. cc The Zo diagram shows that this weakon can have the same composition as the photon, the carrier of the electromagnetic force. Wdu Z o e du e du uu ee dd ss The Electro­Weak Force The Electro­Weak Force 1968 S.Weinberg and A.Salam unified the gauge field theories corresponding to electromagnetism and weak force. They used a theory which employed zero mass carriers. J.Goldstone and P.Higgs have shown since 1960 that when nature breaks symmetry it creates some heavy bosons called the Higgs bosons. Higgs bosons, like the weakons, rotate only to the left and can be attached to weakons. The fact that photons rotate both to the left and right ensure that only weakons will couple with the Higgs bosons and will become heavy. The Higgs bosons were discovered experimentally only in 2000. Spontaneous Symmetry Breaking Spontaneous Symmetry Breaking At high level one “sees” the straight lines in the crystalline structure. At the 10­8 cm level the spherical symmetry of the Na+ and Cl­ ions is evident. Electro­weak force the change of symmetry is at about 10­16 cm. Below 10­16 cm the electromagnetic force and the weak nuclear force look the same, except that weakons rotate only to the left. The structure of salt (NaCl) Observing “Heavy Light” Observing “Heavy Light” Theory Experiment Theoretical mass predictions were: W about 74 GeV, Z about 86 GeV. Until 1980 the heaviest particle observed was Upsilon with 9.4 GeV. 1980 CERN accomplished the building of the first proton­antiproton supercollider with an effective energy of 540 GeV. 1983 two experiments (UA1 and UA5) reported the first findings of the W bosons. Out of about 1 million collisions which were sufficiently violent for the weakons to form the two experiments found only 10 which were associated with the W particles. 1984 UA1 obtained 14 events corresponding to Z and about 100 events corresponding to W and their masses were in 3% agreement with the theory. The Standard Model Re­Written The Standard Model Re­Written Today’s “Standard Model” is based on the electro­weak theory and on quantum chromodynamics. This theoretical model explains well the experimental results. But the Standard Model is not a final unified theory. A final unified theory would probably have a new symmetry which incorporates all the known particles. This theory should explain many numbers that the physicists were not able to explain (for instance the masses of all fermions). The total number of parameters in the Standard Model is no less than 27 and it is hard to believe that they are all as fundamental as the speed of light. Limitations of the The Standard Model Limitations of the The Standard Model Not a true unification of all 4 forces Too many parameters The mystery of the 3 generations The model of a generation No explanation for the fact that the differences between the electric charges of various fermions must be integers, while the electric charges can be fractions. No explanation for the fact that the charge of an electron is rigorously equal to that of a proton. No explanation for the fact that colored fermions have fractions of electric charges, while white fermions have integer charges. Theoretical Alternatives Theoretical Alternatives The most successful are: Grand Unification Theories (GUTs) Compounds Models such as the Pati­Salam preons. Super­symmetry (SUSY) Models Superstring Theories How to choose between them ? Theoretical Consistency Experimental verification (predictions) The Pati­Salam Model (I) The Pati­Salam Model (I) Basic idea: Each quark contains two pre­quarks (or preons), one corresponding to the type (u,d,s,c) and the other to its colour (red, green, blue). Leptons were part of the same model, being built with a type preon and a violet preon. The number of bricks was reduced from 16(=4x3+4) to 8. (see next diagram) Integer charges for quarks Disintegration of quarks into leptons. Implications: The Pati­Salam Model (II) The Pati­Salam Model (II) Preons d red u blue s green c violet ured dred cred sred Same for blue and green Neutrino or uviolet Electron or Muonic Neutrino or cviolet sviolet Muon or dviolet The Life of a Proton The Life of a Proton Will an isolated proton live for ever ? Pati­Salam’s model showed that even proton has a finite lifetime: 1031 years. Experiments have tried to see decaying protons but few were successful: Statistical meaning Proton decays through the weak force into a positron and a few neutrinos. In the Pati­Salam model the lifetime of a quark is 10­9 but the chance of all 3 disintegrating in the same time is extremely small. 1977 in a South African gold mine an experiment established a lifetime of 1029 years 1983 in the Mont Blanc tunnel a150 tons detector obtained data corresponding to 1031 years Experiments at the bottom of the Pacific failed to obtain and positive results. GUTs GUTs Put together tables of quarks and leptons together with the carriers of forces in their interaction Use Group Theories (with basic sets, representations and classes) to incorporate all known particles 1973 S.Glashow and H.Georgi propose SU(5), the simplest GUT (shown next) All GUTs allow quarks to desintegrate into leptons through the hyperweak force (X bosons) SU(5) Basic Set SU(5) Basic Set dR dR dG dB e+ G+Zo+ G G XR4/3 XR1/3 dG G G+Zo+ G XG4/3 XG1/3 dB G G G+Zo+ XB4/3 XB1/3 = photon = anti-neutrino e+ XR­4/3 XG­4/3 XB­4/3 Zo+ W­ XR­1/3 XG­1/3 XB­1/3 W+ Zo Another Change of Symmetry Another Change of Symmetry 1974 ­ H.Quinn and collab. calculated that the SU(5) symmetry can be seen at distances below 10­29 cm. Using SU(5) one can describe the structure of matter with the following diagram: SU(5) colour+electroweak colour, weak, elmg. 10­29 cm 10­16 cm Higgs bosons could be created through the change to the SU(5) symmetry and they are responsible for the mass of the X bosons. SU(5) and Experiments SU(5) and Experiments Although we cannot “see” the world at the 10­29 cm level (we would need 1015 GeV projectiles !), we can test SU(5) through the verifications of its predictions. 1977 a CERN experiment showed that the ratio between the masses of the quark b and the tau lepton is larger than 2.5, while SU(5) predicts 3. 1983 the Mont Blanc protons disintegration experiment obtains results which suggest 1031 years as proton’s lifetime but SU(5) predicts 1029 years. Other, more complex GUT’s predict values closer to 1031 years. Super­Symmetry (SUSY) Super­Symmetry (SUSY) SUSY: for each fermion there is a boson with the same properties except the spin and the other way round. SUGRA=SUSY + GUTs + gravitation. It introduced many useful ideas: It deals with all universal forces It introduces the need to work in many dimensions (11 dimensions) All the these particles are organized in symmetric super­multiplets. As no experiment has seen these new particles, SUSY has no real support. More than 4 Dimensions ? More than 4 Dimensions ? Michio Kaku’s “Hyperspace” for a history Visualizing hyperspace: a Sphere in Flatland. Supergravity in 11 Dimensions Supergravity in 11 Dimensions Kaluza­Klein model was applied successfully in 11 dimensions to account for all the particles of SUGRA(8). The metric tensor of this model is shown below. Einstein’s Gravity Maxwell’s Light Gauge Theory Bosons Matter: quarks and leptons Superstrings Superstrings Superstrings correspond to resonances The basic idea is that particles correspond to resonances that correspond to distinct frequencies of vibration of some strings. They are 1020 times smaller than a proton, or 10­36 cm. Superstrings are extremely small Complicated motions No infinities if calculated in 10 dimensions. M Theory M Theory Recently there were 5 competing 10­dimensional superstrings theories Edward Witten proved the 5 theories to be equivalent and related to an 11­dimensional M­Theory The M­theory has similarities with supergravity Contemporary experiments performed with the most powerful supercolliders will try to produce the sparticles predicted by the M­theory The M­theory has not only superstrings, but also 2,3.. etc ­ dimensional branes (multidimensional vibrations) M Theory is our best candidates for the “Theory of Everything” ...
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